1. in cognitive psychology, a mental image or cognition that is ultimately derived from experience but that may occur without direct reference to perception or sensory processes.

2. in the writings of René Descartes and the British empiricist philosophers (see empiricism), a mental event that may correspond to something outside the mind but that is itself the immediate object of thought or perception. Descartes made a celebrated and much disputed distinction between derived ideas and innate ideas. See also association of ideas.

3. in the thought of Greek philosopher Plato (c. 427–c. 347 bce), the abstract or intellectual form of something regarded as its true reality. See Platonic idealism; theory of forms.